This invention relates to a semiconductor high voltage switching system and method for transmitting and distributing electrical power at very high voltage.
The transmission and distribution of electrical power benefits from the use of high power semiconductor thyristor switches for the distribution of high voltage DC and for the commutation of AC at high voltage. To produce the voltage levels needed to provide power at voltage levels above 10 kv and up to 200 k volts or higher it is commonplace to combine a multiple number of Silicon Controlled Rectifiers SCR""s (hereafter referred to as xe2x80x9cthyristorsxe2x80x9d) in series to form a semiconductor xe2x80x9cvalvexe2x80x9d. Conventional high voltage systems are designed using such semiconductor valves. To test the operation of a system using a semiconductor thyristor valve at multi kv voltage levels involves considerable safety considerations and makes the system very expensive to manufacture.
It is therefore the primary objective of the present invention to simplify the design and manufacture of a semiconductor high voltage electrical energy transmission switching system and to provide a high voltage semiconductor commutation circuit for single and/or three phase operation.
It has been discovered in accordance with the principals of the present invention that by forming a series stack of free floating circuit sections each composed of a predetermined number of semiconductor SCR thyristors in series, auxiliary power can be transferred through the stack of circuit sections by incorporating an auxiliary power transformer in each circuit section arranged such that the voltage difference between the power transformer windings applied to the opposite ends of each circuit section will be equal to the maximum voltage across each circuit section of the stack. Using this arrangement the voltage across each section can be limited to a voltage of, e.g. 10 k volts, which reduces the cost of the semiconductor and firing transformer elements and simplifies the testing of the circuit sections during manufacture and use. The number of sections in a stack will depend on the desired voltage to be applied across each section and the line voltage.
Additional benefits include a reduction in the number of spare parts (the switching system for different voltage levels are built from the same building block) and ease of maintenance in that a faulty section need only be replaced.
Each free floating circuit section in each phase may include any number of unidirectionally series connected thyristors. Alternatively, the thyristors in each section may be bi-directionally connected, i.e., connected in an anti-parallel arrangement to allow current to pass in both directions and to permit forced commutation to turn off the thyristors if this is needed. Forced commutation is used to limit the fault current in a section before it rises to a level which will cause substantial damage to circuit breakers and other components (transformer, busses, etc.) elsewhere in the power system and/or to the components in the section. In the preferred forced commutation switch circuit embodiment of the present invention, applicable to single and three phase operation, the number of commutating capacitors in the switch circuit is reduced by a factor of two compared to conventional commutation switching circuits using semiconductor thyristors in which each semiconductor thyristor requires its own commutating capacitor. This is also true for the number of commutation inductors which is also reduced by a factor of two thereby further reducing the cost and weight/size of the section. For three-phase operation, each phase has a multiple stack of identical free floating circuit sections.
The semiconductor high voltage electrical energy transmission switching system of the present invention includes a multiple stack of power semiconductor SCR thyristor circuit sections arranged for single phase or three-phase operation with each section of the stack comprising; a pair of input and output terminals, a plurality of switching thyristors connected in series between the input and output terminals, an auxiliary power transformer having at least three windings with first and second windings interconnecting the input and output terminals of one section to the respective output and input terminals in each adjacent section of the stack such that the voltage difference between said first and second transformer windings is limited to the maximum voltage across said section, an AC/DC power supply connected to a third winding of said auxiliary power transformer and a gate drive circuit for controlling the firing of said switching thyristors in said section.